Process of jacketing fuel elements

ABSTRACT

1. A process of integrally bonding a jacket of zirconium metal to a uranium core comprising applying a layer of iron onto said core, immersing the jacket into molten zinc chloride of a temperature of between about 410* and 460*C. and assembling said core and said jacket while both are immersed in a molten aluminum-silicon alloy.

United States Patent [1 1 Ray et a1.

[ 1 Nov. 26, 1974 PROCESS OF JACKETING FUEL ELEMENTS [75] Inventors: William E. Ray, Schenectady, N.Y.; John D. Sprowl, Opportunity, Wash.

[73] Assignee: The United States of America as represented by the United States Atomic Energy Commission, Washington, DC.

[22] Filed: Oct. 2, 1957 [21] Appl. No.: 687,839

OTHER PUBLICATIONS WAPD-PWR-PMM-60l, Lasco and Belle,- Feb. 1',

1956, in particular pages 4-8. Available from OTS, Dept. of Comm., Washington, 25, DC.

Metallurgy of Zirconium, Lustman and Kerze, McGraw-Hill Book Co., Inc., 1955, Pages 341-347.

BMI923, July 7, 1954, Available from TIS, PO. Box 1001, Oak Ridge, Tenn.

BMI934 Aug. 3, 1954, Available same as BMl-923.

Primary Examiner-Benjamin R. Padgett Assistant ExaminerB. Hunt Attorney, Agent, or Firm-John A. Horan; Arthur A. Churm; Lena R. Ziegler EXEMPLARY CLAIM l. A process of integrally bonding a jacket of zirconium metal to a uranium core comprising applying a layer of iron onto said core, immersing the jacket into molten zinc chloride of a temperature of between about 410 and 460C. and assembling said core and said jacket while both are immersed in a molten aluminum-silicon alloy.

7 Claims, No Drawings PROCESS OF JACKETING FUEL ELEMENTS This invention deals with a process of bonding metal jackets to uranium fuel elements for neutronic reactors and, in particular, to the bonding of jackets of zirconium metal or zirconium-base alloys to uranium cores.

It has been tried heretofore to braze jackets of zirconium or zirconium-base alloys (hereinafter collectively) referred to as zirconium metals) to uranium cores by means of an aluminum-silicon alloy, in particular with the aluminum-silicon eutectic. However, the fuel elements obtained thereby had the disadvantage that the aluminum-silicon alloy combined with the uranium and formed a ternary aluminum-silicon-uranium compound which was rather brittle and which separated from the core during cooling due to the different expansion coefficients of the uranium and the zirconium metal. Consequently, the articles obtained by that process were highly unsatisfactory, their corrosion resistance was poor and became worse under the temperature fluctuations to which they were exposed in a neu tronic reactor.

it was then attempted to apply a layer of lead to the uranium core prior to bonding of the zirconium metal jacket thereto with the aluminum-silicon alloy; however, the lead layer always was flocculent and the bond with the lead-coated uranium core poor.

lt isan object of this invention to provide a process for bonding zirconium metal to uranium whereby all the above-mentioned disadvantages are overcome.

It is thus an object of this invention to provide a process for bonding zirconium metal to uranium by which good wetting of the zirconium metal and the uranium with the aluminum-silicon alloy is obtained.

it is also an object of this invention to provide a prowhich has a relatively high degree of dimensional stability when exposed to temperature fluctuations.

Another object of this invention is to provide a process for bonding a zirconium metal jacket to a uranium core with an aluminum-silicon alloy whereby the brittle uranium-aluminum-silicon compound is not formed.

It is finally also an object of this invention to provide a process for bonding a zirconium metal to uranium for which no especially complicated and expensive equipment is necessary.

It has been found that. if the uranium core was provided with a very thin layer of iron and ifthe jacket was immersed into molten zinc chloride of a temperature up to 460C., both prior to being bonded with the aluminum-silicon alloy. anintegral unit of great durability was obtained. It was also found that the uranium of the core did not diffuse into the thin iron layer and that it thus also did not diffuse into the aluminumsilicon bond; consequently a brittle uraniumaluminum-silicon compound was not formed. A jacketed fuel element of great durability was thus obtained the properties of which were not impaired during use in a reactor where it was subjected to considerable temperature fluctuations, this in spite of the different expansion coefficients of core and jacket materials.

The process ofthe invention thus comprises applying a thin iron layer onto the uranium core to be jacketed with Zirconium metal, immersing the jacket into molten zinc chloride of a temperature up to 460C. whereby a layer of the chloride is deposited on the jacket and assembling the iron-coated uranium core and the zinc chloride-coated jacket of zirconium metal under molten aluminum silicon alloy.

Iron-coating of the uranium core is preferably effected by electroplating. For this purpose the core was first etched anodically; for instance, a solution containing 356 grams of orthophosphoric acid and 40 cc. of hydrochloric acid per liter of solution was found a satisfactory etching medium. The core was then immersed in a 50 percent nitric acid at about 50C. to remove any coating formed; thereafter, the core was rinsed and immersed in the plating electrolyte. A proper plating bath, for instance, contained 375 grams of ferrous ammonium sulfate per liter of water and had a pH value of about 2.5 and a temperature between 50 and 64C. The uranium core was made the cathode while a ferrous metal, such as iron or steel, was used as the anode. Electrolysis was continued until an iron layer of about 0.001 inch thickness had been obtained.

Although not necessary for preparing the article according to this invention, it was found advisable to superimpose a very thin nickel layer on the iron in order to protect the latter from oxidation. A thin flash coat of 0.0001 inch thickness was found sufficient for this purpose. This nickel coat did not form a part of the final article, since it was dissolved when the core was immersed in the aluminum-silicon bond. The nickel layer can be applied by any method known to those skilled in the art.

The jacket or can, as has been mentioned before, may be made of metallic zirconium or of a zirconiumbase alloy. Zirconium alloys containing from about I to 1.5 percent of tin as the main additional component were found particularly well suitable.

Thejacket was advantageously subjectedto a preparatory treatment which, if necessary, comprised the removal of black oxide by vapor blasting and etching in a nitricand hydro-fluoric-acid-containing aqueous solution. The etching step was essential. A suitable composition for the etching solution contained the ingredients in the weight ratio of5 4 l for H O HNO;, HF; an immersion time of about 30 seconds was found to be sufficient. The jacket was then rinsed in water followed by rinsing in acetone; finally it was dried. Etching should not be carried at earlier than ten minutes, but preferably only seconds, prior. to coating with zinc chloride.

Coating of the jacket with zinc chloride was then carried out to improve its wettability with the aluminumsilicon alloy. The temperature for the zinc chloride bath is critical; it should not be higher than 460C. and preferably range between 410 and 460C. Above the temperature of 460C.. the deposit. formed on the zirconium was not zinc chloride but metallic zinc; in the case of a zinc layer, the aluminum-silicon bond always separated from the core during use in the reactor; in

other words, the bond was unsatisfactory. The zirconium jacket was preferably immersed in the zinc chloride for a period of between one-half and one minute; for instance, immersion for 1 minute in a bath of 410C. yielded satisfactory results. The jackets thus coated were then air-cooled and rinsed with water to remove any excess zinc chloride.

The cans thus prepared were then immersed in aluminum-silicon alloy, preferably the eutectic, which had a temperature of between 615 and 635C. The immersion time ranged preferably between 30 seconds and 2 minutes.

The iron-plated uranium core was first preheated in the so-called duplex bath which was a 60-kw. resistance pot furnace containing a layer of the aluminumsilicon eutectic over a layer of lead. The core was immersed into the lead layer and agitated therein for about 30 seconds. Then the core was withdrawn through the aluminum silicon layer and carried with tongs to the canning bath. The canning pot containing eutectic aluminum-silicon alloy was heated in a 60-kw. induction furnace to between 615 and 635C. lmmersion of the core and the jacket were timed so that the core was in the canning bath for 1 to 2 seconds when the jacket had been therein for from 30 seconds to 2 minutes. The core was then inserted into the can in the bath and allowed to seat there under its own weight; this required from 30 seconds to 2 minutes. A cap which had been pretreated together with the jacket by the same method and which consisted of the same zirconium metal as the jacket was then placed on top of the core. The assembly obtained was slowly quenched in water starting at the bottom and gradually immersing it up to its top. The assembly was finally etched on the outside with a 5 percent caustic solution, e.g.. with a solution of sodium hydroxide of 65C., or else it was machined in a lathe to remove aluminum-silicon bond adhering to the surface.

The process of this invention can be applied to any types of fuel elements in which a uranium core is canned or jacketed with a zirconium-base metal. For instance, fuel elements were assembled using a core of a-rolled and B-heated uranium 1.375 inches in diameter and 4.2 inches long and ajacket having a wall thickness of 0.020 inch and an outer diameter of 1.440 inches. The cans had a dished" base of a radius of about 6 inches to which a 0.060 inch thick zirconium metal wafer had been welded to produce a flat-bottom can. The caps in this'can had a diameter of 1.380 inches and were 0.220 inch thick.

Of course. the above dimensions are only given by way otexample. since a uranium slug of any size can be canned by the process of-this invention. However, it is important that the clearance between the core and the jacket is sufficient so that the core does not have to be forced into the jacket whereby the thin iron coat may be damaged so that the quality of the bonding would be impaired. It was found that the clearance between jacket and core should be at least 0.02 inch and range up to 0.05 inch.

' The finished articles canned by the process ofthis invention were then subjectedto a number of tests to examine the quality of the bond. the durability of the bond after temperature fluctuations and the dimensional stability of the fuel'elements. The adherence was examined first'by looking for blisters on the can wall which were always a sign of poor adherence. Furthen more, the can surface was tapped with a metal bar the sound thereby being indicative of the quality of the bond; a ringing sound always revealed a satisfactory bond. Moreover, the jackets were mechanically stripped from the core and then inspected to discover small unbonded areas.

In order to test for durability of the bond and for dimensional stability after temperature fluctuations as they occur in neutronic reactors, the jacketed slugs were heated to 350C. in a muffle furnace having a temperature of 550C; the temperature of the slug was measured by a thermocouple inserted between the can and the core. When a temperature of 350C. was reached, the slug was removed from the furnace and quenched in water of room temperature. The heating time usually was about 9 minutes; 100 cycles of heating and quenching were carried out. The slugs thus cycled were then sectioned for metallographic examination.

All of the slugs jacketed by the process of this invention showed good adherence in contradistinction to those which were bonded with aluminum-silicon alloys without a layer of iron or those which had a layer of lead instead of iron. However, it was found that, among the iron-coated slugs, only those were satisfactory in which the temperature of the zinc chloride had been kept at a maximum of 460C; when the zinc chloride had a temperature of above 460C, the aluminumsilicon bond separated after cooling thereby making the slugs unsatisfactory. The dimensional changes after thermocycling were of nonobjectionable magnitude in the slugs prepared by the process of this invention.

It will be understood that this invention is not to be limited to the details given herein, but that it may be modified within the scope of the appended claims.

We claim:

I. A process of integrally bonding a jacket of zirconium metal to a uranium core comprising applying a layer of iron onto said core, immersing the jacket into molten zinc chloride of a temperature of between about 410 and 460C. and assembling said core and said jacket while both are immersed in a molten aluminum-silicon alloy.

2. The process of claim 1 wherein the core and the jacket are etched with acid prior to iron-coating and zinc chloride-coating, respectively.

3. The process of claim 1 wherein said iron layer is about 0.001 inch thick.

4. The process of claim 1 wherein the zirconium metal is elemental zirconium.

5. The process of claim 1 wherein the zirconium metal is a zirconium-tin alloy containing from 1 to 1.5 per cent by weight of tin.

6. The process of claim 1 wherein the aluminumsilicon alloy is the eutectic and has a temperature of between 615 and 635C.

7. A process of integrally bonding a jacket of zirconium metal to a core of uranium metal, comprising applying a layer of iron onto said core; applying a layer of nickel onto said layer of iron; immersing the jacket in an etching solution of a hydrochloric acid nitric acid mixture; promptly, after etching, immersing the jacket in molten zinc chloride having a temperature of between 410 and 460C; immersing the iron-nickelcoated uranium core in molten lead and immediately thereafter in molten aluminum-silicon eutectic; assembling the core and the jacket while both are immersed in molten, aluminum-silicon eutectic; and quenching the assembly obtained slowly and gradually starting from the bottom and proceeding upwardly. 

1. A PROCESS OF INTEGRALLY BONDING A JACKET OF ZIRCONIUM METAL TO A URANIUM CORE COMPRISING APPLYING A LAYER OF IRON ONTO SAID CORE, IMMERSING THE JACKET INTO MOLTEN ZINC CHLORIDE OF A TEMPERATURE OF BETWEEN 410* AND 460*C. AND ASSEMBLING SAID CORE AND SAID JACKET WHILE BOTH ARE IMMERSED IN A MOLTEN ALUMINUM-SILICON ALLOY.
 2. The process of claim 1 wherein the core and the jacket are etched with acid prior to iron-coating and zinc chloride-coating, respectively.
 3. The process of claim 1 wherein said iron layer is about 0.001 inch thick.
 4. The process of claim 1 wherein the zirconium metal is elemental zirconium.
 5. The process of claim 1 wherein the zirconium metal is a zirconium-tin alloy containing from 1 to 1.5 per cent by weight of tin.
 6. The process of claim 1 wherein the aluminum-silicon alloy is the eutectic and has a temperature of between 615* and 635*C.
 7. A process of integrally bonding a jacket of zirconium metal to a core of uranium metal, comprising applying a layer of iron onto said core; applying a layer of nickel onto said layer of iron; immersing the jacket in an etching solution of a hydrochloric acid - nitric acid mixture; promptly, after etching, immersing the jacket in molten zinc chloride having a temperature of between 410* and 460*C.; immersing the iron-nickel-coated uranium core in molten lead and immediately thereafter in molten aluminum-silicon eutectic; assembling the core and the jacket while both are immersed in molten aluminum-silicon eutectic; and quenching the assembly obtained slowly and gradually starting from the bottom and proceeding upwardly. 